1. Field of the Invention
The present invention relates to the construction of a shaft for use in winding or rewinding webs, such as strips of paper, bands of woven cloth, sheets of metal.
2. Description of the Prior Art
The construction of the conventional web winding or rewinding shaft is shown in FIG. 1. As seen from FIG. 1, the shaft construction includes a central shaft 1 having cone-shaped members 2, 2 at the opposite ends thereof. The cone-shaped members 2, 2 are capable of axial sliding movement. A winding pipe 4, which is usually made of paper, is removably mounted between and held by the cone-shaped members 2, 2 so that the winding pipe 4 can have the coaxial relationship with the center shaft 1. The construction shown in FIG. 1 is typically used when a strip of paper is rolled.
According to the shaft construction as shown in FIG. 1, when a winding pipe 4 is to be mounted to or removed from the center shaft 1, the cone-shaped members 2, 2 must also be removed. This requires the mounting or removing steps for the cone-shaped members 2, 2, which reduces the working effieciency.
More recently an alternative shaft construction has been developed and actually used. This shaft construction employs an air shaft, as shown in FIG. 2. The air shaft construction includes a hollow shaft 5 having a set of lugs 6, 6 extending radially through its wall, and a rubber tube 7 mounted inside the hollow shaft 5. Each of those lugs 6, 6 can project through the hollow shaft outwardly or retract inwardly, under the action of the rubber tube 7. When the rubber tube 7 is supplied with a compressed air, and becomes inflated, it actuates the lugs 6, 6 so that they can move radially outwardly toward a paper winding pipe 4 mounted outside the hollow shaft 5, until they can engage the winding pipe 4. In this way, the winding pipe 4 can be held firmly so that it can be maintained in its coaxial relation to the hollow shaft 5. Usually, the set of lugs 6, 6 consist of four lugs which are mounted at equal intervals around the hollow shaft 5. For a given length of the air shaft construction, one set of the four lugs arranged as described above is arranged at an interval of every one meter along the total effective length of the air shaft construction.
Although the conventional air shaft construction may provide an advantage over that shown in FIG. 1 in that the steps involved in mounting or demounting a winding paper-pipe have been eliminated, and therefore may have been used in a wide range of applications, it has several problems as follows:
(1) As a web, such as a strip of paper or a sheet of metal, is being wound around a winding paper-core, its weight will be increasing to the extent that the holding force of the lugs cannot sustain the overall weight of the web. In that event, the coaxial relationship between the hollow shaft 5 and winding paper-core 4 cannot be maintained, causing the winding paper-core 4 to deviate from the hollow shaft 5, as shown in FIG. 3. When the winding paper-core 4 deviates from the center, the tensional force will be given unevenly to the web being wound. Thus, the result may contain irregularly wound portions of the web, totally or partially.
This may be explained by determining the approximate value of the holding or pushing force that may be provided by the lugs against the winding paper-pipe or core. Then, it is assumed that each one lug has an area of 16 cm.sup.2 (2 cm wide and 8 cm long) to be contacted by the rubber tube, and a compressed air of 4 kg/ cm.sup.2 is delivered into the tube. The resulting holding or pushing force of the single lug may be obtained as a product of the two parameter values given above, that is 64 kg. Thus, the total holding or pushing force provided by the four lugs that are arranged every one meter along the length of the hollow shaft 5 may be obtained as follows: EQU 64 kg.times.4 (lugs)=256 kg
For a web of paper, it is assumed that it has been wound around its core 4 to a diameter of one meter. Its weight (per meter of length) is substantially equal to 500 kg. It may readily be understood from the above equation that the four lugs cannot overcome this weight. In other words, the total holding force of the lugs is not sufficient to prevent the web core from deviating from the center. If the web is a cloth or metal sheet, its total weight per meter of length become much greater, also causing the problem of deviation.
(2) If the deviation occurs as described above, and the hollow shaft is still rotating in that condition, a given lug which is being forced back inwardly under load is moving toward the position occupied by the preceding adjacent lug. The other lugs behave similarly, moving to the position occupied by the preceding adjacent lug. In this manner, each lug is projecting outwardly and retracting inwardly. As this movement is repeated, the portion of the rubber tube that in contacted by those lugs, including the region surrounding that portion, will eventually become worn due to the friction between the rubber tube and lugs, and will also become deformed under the repeated loads, causing the fatigue of the rubber material that may break the rubber tube. This may cause air leaks to occur from the rubber tube.
(3) The pressure of the lugs upon the winding paper-core or pipe is very large (such as approximately 64 kg), as determined from the above equation for the holding or pushing force. For this reason, the winding paper-pipe must have both the thickness and mechanical strength sufficient to sustain or overcome that pressure. Usually, this paper winding core or pipe is supplied for a one-time use, rather than for multiple-time use. Thus, once used, it will become useless. In this respect, it is not economical.